1. Field of the Invention
The present invention concerns a method for recovering viscous petroleum from subterranean viscous petroleum containing formations, and especially those formations which are relatively thick and which may have one or more high permeability strata which makes normal recovery methods ineffective because of channeling of the recovery fluid through the high permeability strata, thereby bypassing substantial amounts of petroleum. More specifically, this method employs a combination downward gas displacement, thermalsolvent method whereby viscous petroleum may be recovered from thick formations having one or a multiplicity of horizontally oriented thief zones or high permeability strata.
2. Description of the Prior Art
There are many subterranean, petroleum-containing formations in various parts of the world from which petroleum cannot be recovered efficiently by conventional means because the petroleum is too viscous to flow or be pumped. The most extreme example of viscous petroleum-containing formations are the so-called tar sands or bituminous sand deposits. The largest and most famous such deposit is the Athabasca Tar Sand Deposit in the Northeastern part of the Province of Alberta, Canada, which contains over 700 billion barrels of petroleum. Other extensive deposits are known to exist in the Western United States and in Venezuela, and other smaller deposits exist in Europe and Asia.
Tar sands are defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum constituent of tar sand deposits is a highly bituminous petroleum. The sand is generally fine quartz sand coated with a layer of water, with the viscous bituminous petroleum occupying most of the void space around the water wetted sand grains. The balance of the void space is filled with connate water, and some deposits contain small volumes of gas such as air or methane. The sand grains are packed to a void volume of about 35 percent, which corresponds to about 83 percent by weight sand. The balance of the material is bituminous petroleum and water, and the sum of bituminous petroleum and water is fairly consistently about 17 percent by weight, with the bitumen portion thereof varying from 2 percent to about 16 percent. In general, one of the characteristics of tar sand deposits which differs considerably from more conventional petroleum-containing formations is the absence of consolidated mineral matrix within the formation. While the sand grains are in contact, they are generally uncemented. The API gravity of bituminous petroleum present in tar sand deposits ranges from about 6 to about 8 and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027.
Methods described in the prior art for recovering bituminous petroleum from tar sand deposits include strip mining and in situ separation processes. Most of the recovery which has been accomplished up to the present time has been by means of strip mining, although this is economically feasible only when the ratio of overburden thickness to tar sand deposit thickness is around one or less. The majority of the petroleum known to exist in tar sand deposits is in formations too deep to be economically suitable for strip mining, so there is a serious need for some type of in situ recovery process wherein the bituminous petroleum is separated from the sand in the formation and recovered therefrom through a well or other production means drilled into the deposit.
In situ recovery processes described in the literature may be categorized as thermal techniques, including fire flooding or in situ combustion, and steam flooding, and emulsification drive processes, and combination of these processes. Any in situ recovery process must accomplish two functions: the viscosity of the crude petroleum must be reduced to a sufficiently low level that it is mobile, and sufficient driving energy must be applied to the crude petroleum to induce it to move through the formation to the production well or other means for transporting it to the surface of the earth.
Most of the in situ recovery processes discussed above involve the injection of a thermal fluid such as steam or air for in situ combustion into one well drilled into the formation, to move generally horizontally through the formation into a remotely located production well. If the tar sand deposit is relatively thick, i.e. in the range of 50 feet or more in thickness, and particularly if there are present in the tar sand deposits, one or more thief zones or intervals having permeabilities substantially greater than the permeability of the balance of the formation, the injected thermal fluid tends to channel through the high permeability strata, thereby bypassing substantial amounts of the petroleum-containing formation both above and below the high permeability strata. This results in the phenomena referred to as poor vertical conformance, and can reduce the volume of formations swept by the displacement fluid to a relatively small number, i.e., 10 to 30 percent of the total volume of the formation.
U.S. Pat. No. 3,838,738, Oct. 1, 1974, Redford et al. describes a method for recovering viscous petroleum from a subterranean formation involving forming a communication channel deep in the formation and injecting a thermal fluid and a solvent to cause vaporization of the solvent upward into the formation so the viscous petroleum is diluted and flows downward into the channel by force of gravity, from which it can be displaced to the surface of the earth.
Many prior art techniques deal with the problem of permeability thief zones in formations, but most are concerned with methods for treating the portion of the formation immediately adjacent the injection well to reduce its permeability. While this is initially effective, injected fluids quickly channel around the treated portion of the formation back into the thief zone and result in poor vertical conformance and poor displacement and consequently poor sweep efficiency.
In view of the foregoing discussion, it can be appreciated that there is a substantial need for a method applicable to thick, viscous petroleum-containing formations which contain one or more strata having permeability substantially greater than the remaining portions of formation, so as to effectively recover viscous petroleum without bypassing substantial portions of the formation with the injected recovery fluid.